The immune system, based on several kinds of specialized blood white cells, is a highly specific defense system that recognizes, eliminates and remembers foreign macromolecules and cells. While functioning properly, it can distinguish between "self" and "non-self" (foreign) materials. For example, it views tumor cells as non-self and hence attacks them, protecting animals against cancer-causing tumor cells as it protects against other invading macromolecules.
An antigen is a foreign substance that when introduced into an animal with a functioning immune system, can elicit a specific immune response such as the one mentioned above. Once activated the immune response involves, among other things, production of antibodies in the circulation system specific to that antigen. There are five distinct classes of antibodies which are also called immunoglobulins. The most abundant is IgG. The other four are IgM, IgA, IgD, and IgE. These antibodies combine with the antigen and act to neutralize or counter the effects of the antigen introduced into the animal. They accomplish this result by binding to the antigen thereby neutralizing it and preventing it from binding to other specific cell receptors.
The immune system can be used not only to fight off pathogenic antigens or harmful foreign molecules, but can be manipulated in order to elicit favorable responses which are not naturally occurring. For example, naturally occurring proteins in an animal can be neutralized via introduction of antibodies specific to that protein thereby neutralizing that protein's normal physiological affect on the animal's system.
There are several ways in which an animal becomes immune responsive. For example, some antibodies are able to traverse the placenta from a mother's circulation to that of her fetus. As a result, the progeny of that mother receives natural immune protection by "inheriting" the mother's own antibodies before birth.
A second way to elicit an immune response is through introduction of an antigen into one animal, resulting in that animal developing specific antibodies to that antigen. These antibodies can then be isolated from the animal and introduced into a second animal resulting in the second animal having antibody that can bind the specific antigen.